When lithium ion batteries and lead-acid batteries are used in combination, it is necessary to adjust the operating potential of the lithium ion battery to that of the lead-acid battery. An average operating potential of about 13.5 V is achieved by connecting in series, six batteries, in which a lithium-nickel-cobalt-manganese composite oxide is used for the positive electrode, and a spinel type lithium-titanium oxide is used for the negative electrode. Such lithium ion batteries can be used in combination with lead storage batteries. Lithium-titanium composite oxide undergoes only a slight volume change accompanying charge and discharge cycles and so has excellent cycle characteristics. Additionally, lithium metal is hardly deposited in a lithium absorption and release reaction of lithium-titanium composite oxide, in principle, and thus, performance degradation of batteries using lithium-titanium composite oxide is small, even after repeating charge and discharge cycles in large currents.
As such, when lithium ion batteries and lead-acid batteries are used in combination in vehicles or the like, low-temperature output performance at about 13 V, which is the working voltage of lead-acid batteries, is demanded. Unfortunately, the above batteries have a state of charge of 10 to 30% when a battery pack, in which six batteries are connected in series, is adjusted to 13 V. Therefore, there is a problem that the low-temperature output performance is low.